Blender Git Loki

Fixes for Time Marker support in Action window;

- now draws on correct height
- selection is clipped correctly (did entire height)
- made showing scene markers default

Actually this implementation should have been a patch review... it was
done with entirely duplicating the code from timeline.c, whilst re-use
and generalizing markers has a preference. That way markers can become
supported in any time-based editor.

Will send an extensive review to Joshua Leung :)